Variable waveguide



Jan. 14, 1964 c. a. WATTS, JR 3,118,113

VARIABLE WAVEGUIDE Filed May 27, 1960 2 Sheets-Sheet 1 IN V EN TOR.

A TTORNEYS United States Patent 3,118,118 VARIABLE WAVEGUIEEE Chester B.Watts, ha, Alexandria, Va., assignor to Scanwell Laboratories, 1112.,Springfield, Va., 2 corporation of Virginia Filed May 27, 1960, Ser. No.32,393 3 Claims. (Cl. 33331) This invention relates to the transmissionof radiofrequency or microwave energy in waveguides, and morespecifically to the rapid control of the phase or the amplitude of thefield associated with such energy.

It is sometimes desirable, in microwave systems, to be able, in responseto a control signal, to change the RP phase of a signal in a waveguidewithout at the same time causing appreciable amplitude change. This hasbeen done in a number of difierent ways by devices known customarily asvariable phase-shifters. It is ordinarily pos sible to classify suchdevices either as electro-mechanical or as non-mechanical in nature. Theelectro-mechanical types are characterized by relatively high stabilityand good linearity but usually respond rather slowly to a controlsignal.

It is an object of this invention to provide an electromechanical typeof variable phase-shifter wherein the required mechanical motion issmall and the mass of the moving part is also small, thereby permittingrelatively rapid response to a control signal.

Another object of this invention is to provide an adjustable band-passwave filter wherein the required mechanical motion is small and the massof the moving part is also small, thereby permitting rapid variation ofthe cut-off frequencies in response to a control signal.

An embodiment of this invention provides a length of wave-guide ofrectangular cross section wherein one wall carries a series of inwardprojections. The opposite wall of the wave-guide is a relatively thinmetal diaphragm which is permitted limited transverse motion. Motion isimparted to the diaphragm by one or more driving coils in permanentmagnet fields after the fashion of electrodynamic loud speakers, by asuitable control signal.

FIG. 1 is an elevation view, with parts in section, of a complete devicewhich embodies this invention;

FIG. 2 is a sectional view, taken along the plane 2-2 in FIG. 1;

FIG. 3 is a graph showing typical attenuation and phase characteristics;and

PEG. 4 is a diagrammatic representation of the controlling circuit.

A device which embodies this invention may be described in more detailwith refernce to FIGS. 1 and 2, as follows: A waveguide is shown in theform of a pipe 1 of rectangular cross section bounded by three rigidwalls 2, 4, 6 and one movable wall in the form of a diaphragm 8. Thediaphragm may be secured to the lower edges of the side walls 4 and 6,and to a pair of end members 7, by a series of screws 11 passing throughaligned holes in a ring 13 and through the end members 7 and side wallsand 6. End flanges 10 and 12 are provided for joining this device withother waveguide components. The top wall 2 carries a periodicdelaying-structure cor nprising a number of parallel finlike projections14. At the ends, projections 16, =18, and 19 have progressively reducedheight to provide a less abrupt impedance transition into the periodicdelay-structure. A stiif, lightweight beam is cemented, spot-welded orotherwise suitably fastened to the movable portion 9 of the diaphragm 8.A pair of driving coils 2 2. and 24 are similarly fastened to the bottomwall of the beam 2% and are supported in the gaps between the poles of apair of field magnets 26 and 28. The latter are fastened by screws 2? tothe base bracket 30 having legs on the ends thereof fastened by screws31 to the end members 7. Dust-covers 32 and 34 close the sides of thedevice. The dust-covers are fastened to the rigid wall 2 and to the basebracket 39 by a series of screws 33.

It is well known 112 that a waveguide containing a periodicdelay-structure of the type described has a bandpass transmissioncharacteristic, illustrated typically in FIG. 3. The symbols used inFIG. 3 may be defined as follows:

u attenuation component of transmission fi=phase component oftransmission f=applied frequency f lower cut-01f frequency f =uppercut-01f frequency The pass band of interest lies between the frequenciesi and f which are functions of various dimensions of the structure. Inthis device, one of these dimensions is made variable, namely, the gapbetween diaphragm 8, which is the bottom wall of the waveguide, and thebottom ends of projections 14 which form the periodic delaystructure. Ifthe gap is reduced by an upward displacement of diaphragm 3, the resultis that the pass band is moved lower in frequency. This is accompaniedby a corresponding increase in phase-shift at any given operatingfrequency within the pass band. The device thus functions as a variablephase-shifter in response to displacement of diaphragm 8. Displacementis produced by the forces resulting from interaction of control currentsapplied to the coils 22 and 24 with the fields of permanent magnets 26and 28.

The possibility of achieving very quick response in this device may beexplained as follows: It will be readily understood that the undampednatural period of the movable mechanical system is given simply by theformula T :the undamped natural period m=the total mass of the movingparts k=the total spring stiffness constant of the diaphragm.

wherein The total mass, m is made up of the driving coils 22 and 24, thestiffener beam 2%, and the movable portion 9 of the diaphragm 8. Theseparts can be designed to have relatively low mass, thus contributing toa small value of natural period T. The spring stiffness, k, may bedefined as follows wherein f=the force applied by the driving coils xthe displacement of the diaphragm B rnagnetic flux density L=length ofwire in the driver coil windings i: coil current.

1 Radio Research Laboratory Staff Very HighFrequency 'gtgchgquggiMcGraw-Hill Book 00., New York, N.Y., Section ..4 '7.

-18. 13, Colin. Design Relations for the Wide-Band Waveguide Filter,Proc. I.R.E., vol. 38, p. 799; July, 1950.

of high flux density, B, and coil current, 2', as large as compatiblewith heating and mechanical limitations. These factors also contributeto a larger value for the diaphragm stillness, k, thus further reducingthe natural period, T.

4 is a diagrammatic representation or" a control circuit, including asource of 11C. electrical energy, such as a battery 36, a conductor 37to a variable rheostat 38, a conductor 4 from the movable arm 42 of therheostat to the driving coils 22 and 24 which may be connected inseries, and a conductor 44 to the negative terminal of the battery 36.Operation of the movable arm 42 of the variable rheostat is effective tovary the amount of electrical current passing through the driving coils22 and 24 and consequently vary the position of the movable portion 9 ofthe diaphragm a relative to the pro jections 14. If desired, aconventional reversing switc can be added to control the direction ofmovement of the driving coils 22 and 24 in an obvious manner.

While, in the illustrated embodiment, the movable flexible wall 3 isdisposed opposite the Wall'Z carrying the projections 14, it should beunderstood that the device would operate the accomplish the same resultsif the Wall 8 were rigid and one or both of the walls 4 and 6 wereflexible.

The device has been described from the standpoint of rapid phasecontrol. However, the fact that the device has a movable band-passcharacteristic implies that it can be used also as an amplitudemodulator or as a high speed switch by varying a cut-off frequency froma point above to a point below a chosen band of signal frequencies.

While a single specific embodiment of the invention has been shown anddescribed herein, it is to be understood that other forms may beresorted to within the scope of the appended claims.

I claim:

1. A variable waveguide comprising, a tube having three rigid electricalconducting walls and one flexible electrical conducting wall section,one of said rigid conducting walls having a series of internalprojections, said flexible Wall section being sealed around its entireperiphery to said tube, driving means including at least one coil, fixedon the outer side of said wall section and arranged to impart parallelin and out motion to at least the principal central portion of said wallsection and nonpara-llel in and out motion to the minor non-centralportion of said Wall section, said coil being in the flux field of apermanent magnet mounted on the outside of said tube, and means forsupplying a signal current to said coil.

2. A variable waveguide as defined in claim 1 wherein said flexible wallsection comprises a thin and generally fiat member secured throughoutits periphery to said rigid walls, and corrugations extending aroundsaid fiat member adjacent but inwardly of its periphery, stiffeningmeans secured to said member inwardly of said corrugations said drivingcoil being secured to said flat member inwardly of said corrugations.

3. A variable waveguide as defined in claim 2 wherein said stiffeningmeans is secured to said member on the outer surface thereof, therebeing at least two of said coils symmetrically arranged on saidstiffening means, said means for supplying a signal current beingarranged to supply signal current to both said coils in the same phase.

Re erences (Iited in the file of this patent UNITED STATES PATENTS2,238,117 Koch Apr. 15, 1941 2,474,688 Please June 28, 1949 2,567,748White Sept. 11, 1951 2,577,146 Norton Dec. 4, 151 2,697,209 Sichak Dec.14, 1954 2,708,236 Pierce May 10, 1955 2,734,171 Heins Feb. 7, 19562,825,841 Convert Mar. 4, 1958 2,892,157 Schlandsker June 23, 1959 OTHERREFERENCES Article: Green, Corrugated-Waveguide Band-Pass Filters,Electronics 24: 117l19, July 1951.

1. A VARIABLE WAVEGUIDE COMPRISING, A TUBE HAVING THREE RIGID ELECTRICALCONDUCTING WALLS AND ONE FLEXIBLE ELECTRICAL CONDUCTING WALL SECTION,ONE OF SAID RIGID CONDUCTING WALLS HAVING A SERIES OF INTERNALPROJECTIONS, SAID FLEXIBLE WALL SECTION BEING SEALED AROUND ITS ENTIREPERIPHERY TO SAID TUBE, DRIVING MEANS INCLUDING AT LEAST ONE COIL, FIXEDON THE OUTER SIDE OF SAID WALL SECTION AND ARRANGED TO IMPART PARALLELIN AND OUT MOTION TO AT LEAST